This invention relates to acoustic ink printing, and more particularly to a method and apparatus that allows an acoustic ink printer to operate at operational speeds greater than previously achievable and, which extends the ink types which may be used with the acoustic ink printer, while at the same time ensuring appropriate ink drop ejection directionality to achieve desired output printing.
It has been shown that acoustic ink printers which have printheads comprising acoustically illuminated spherical or Fresnel focusing lenses can print precisely positioned picture elements (pixels) at resolutions that are sufficient for high-quality printing of complex images.
Although acoustic lens-type droplet emitters currently are favored, there are other types of droplet emitters which may be utilized for acoustic ink printing, including (1) piezoelectric shell transducers, such as described in Lovelady et al., U.S. Pat. No. 4,308,547, and (2) interdigitated transducers (IDTs), such as described in commonly assigned U.S. Pat. No. 4,697,195. Furthermore, acoustic ink printing technology is compatible with various printhead configurations; including (1) single emitter embodiments for raster scan printing, (2) matrix configured arrays for matrix printing, and (3) several different types of page and width arrays, ranging from (i) single row sparse arrays for hybrid forms of parallel/serial printing, and (ii) multiple row staggered arrays with individual emitters for each of the pixel positions or addresses within a page width address field (i.e., single emitter/pixel/line) for ordinary line printing.
For performing acoustic ink printing with any of the aforementioned droplet emitters, each of the emitters launches a converging acoustic beam into a pool of ink, with the angular convergence of the beam being selected so that it comes to focus at or near the free surface (i.e., the liquid/air interface) of the pool. Moreover, controls are provided for modulating the radiation pressure which each beam exerts against the free surface of the ink. That permits the radiation pressure from each beam to make brief, controlled excursions to a sufficiently high pressure level to overcome the restraining force of surface tension, whereby individual droplets of ink are emitted from the free surface of the ink on command, with sufficient velocity to deposit them on a nearby recording medium.
An attraction of acoustic ink printing is the ability to control droplet size based on the frequency of the signal provided, rather than relying on the size of the nozzle emitting the droplet. For example, an acoustic ink printer may emit droplets which are a magnitude or more smaller than the acoustic ink printhead openings. On the other hand, conventional ink jet printing requires a minimization of the nozzle itself to obtain smaller droplets.
Ideally, in an acoustic ink printer, the acoustic wave propagates in a direction perpendicular to the air-ink surface. The acoustic wave causes a droplet to be ejected in a direction which is parallel to the direction of the acoustic wave propagates. Thus, ideally the droplet is ejected in a direction perpendicular to the air-ink interface. To achieve high-quality printing, it has been considered necessary that the direction of droplet ejection must be the same for all ejectors across a printhead. Very slight misdirections cause droplets to land on a substrate, e.g., paper, at a location distant from their intended locations.
Typically, a 1 mm gap separates the air-ink interface from the substrate. A droplet ejected one degree off from the ideal ejection direction is displaced 17.5 .mu.m from its intended location on the substrate. For a 1200 spi (spots per inch) printer, this displacement constitutes 80% of one pixel. Thus, in existing systems it has been a high priority to ensure that the direction of ejection of the droplets must be controlled very closely to achieve high-quality printing.
A common cause of misdirectionality is that waves generated from a previous droplet ejection have not settled sufficiently before the next droplet is ejected.
Thus, for conventional acoustic ink printing systems, a design constraint is the time between droplet ejection must be sufficient so as to ensure settling of the surface acoustic waves so that the next ejected droplet maintains good directionality as it moves toward the substrate. In this regard, time required for acoustic waves to settle is a fundamental limit on the print speed of an acoustic ink printer.
Ink settling time decreases with increased ink surface tension. Thus, aqueous inks in acoustic ink printing tend to be high-surface tension inks.
Substantial effort has been directed to improving the directionality of the ink droplets ejected from an acoustic ink ejector, and to designs which decrease the ink settling time, in order to increase printing speed. Examples of efforts in these areas are described in many commonly assigned U.S. patents including: U.S. Pat. No. 4,697,195 entitled Nozzleless Liquid Droplet Ejectors; U.S. Pat. No. 4,748,453 Entitled Spot Deposition for Liquid Ink Printing; U.S. Pat. No. 4,748,461 Entitled Capillary Wave Controllers for Nozzleless Droplet Ejectors; U.S. Pat. No. 4,719,480 entitled Spatial Stabilization of Standing Capillary Surface Waves; U.S. Pat. No. 4,719,476 entitled Spatially Addressing Capillary Wave Droplet Ejectors and the Like; U.S. Pat. No. 5,919,354 entitled Method and Apparatus for Suppressing Capillary Waves in an Ink-jet Printer; U.S. Pat. No. 5,229,793 entitled Liquid Surface Control with an Applied Pressure Signal in Acoustic Ink Printing; U.S. Pat. No. 5,216,451 entitled Surface Ripple Wave Diffusion in Apertured Free Ink Surface Level Controllers for Acoustic Ink Printers; U.S. Pat. No. 5,450,107 entitled Surface Ripple Wave Suppression by Anti-reflection in Apertured Free Ink Surface Level Controllers for Acoustic Ink Printers; U.S. Pat. No. 5,629,724 entitled Stabilization of the Free Surface of Liquid; U.S. Pat. No. 5,808,636 entitled Reduction of Droplet Misdirectionality in Acoustic Ink Printing; U.S. Pat. No. 5,870,112 entitled Dot Scheduling for Liquid Ink Printers, all hereby incorporated by reference.
Various ones of the above references specifically note the importance of directionality in acoustic ink printing as well as the importance of surface waves in achieving desired directionality.
However, the ink ejection process in these documents, as well as the conventional state of the art, is to provide a sequential burst of ink droplets when printing to a substrate or to generate a checkerboard type print output.
Checkerboard printing is a two pass process, wherein each pass prints a portion of the pixels in a dot pattern known as a "checkerboard" pattern. In this type of two pass printing, a first pass of the printhead carriage prints a swath of information in which odd numbered pixels of odd numbered rows or scanlines and even numbered pixels of even numbered rows or scanlines of a bitmap are printed. In a second pass of the carriage printhead, the complementary pattern consisting of even numbered pixels in odd numbered rows and odd numbered pixels in even numbered rows is printed. By printing in two passes, the ink printed in the first pass has time to dry partially before the ink from the second pattern is deposited.
The cited material does not however, recognize the potential benefits of relaxing ink ejection constraints when in a dark/shadow image area, and thus does not apply this understanding through the use of specialized filler patterns which adjust ink droplet ejection.
While other printing arts such as those using half-toning concepts do include the concept of staggered or varying print sequences (i.e., as in the generation of half-tone cells,) such use is directed towards achieving a desired tone scaling. In other words, in half-toning it is desirable to provide smooth transition variations during printing and that is where the half-toning print sequences are directed. However, the concepts of the present invention are specifically directed to directionality and are not concerned with such tone scaling concepts.
The present invention departs from conventional acoustic ink printer designs which have constraints on firing frequency due to the need to allow an ink surface to settle sufficiently before a next ejection. The invention also takes advantage of the inventor's understanding that constraints against misdirectionality within dark or shadow areas of an image may be relaxed in a beneficial manner. It is noted the constraints of existing systems result in an inherent limitation on the speed with which a device may print. For example, existing systems based on aqueous inks, are known to have an upper level operating frequency of 48 kHz.
In consideration of the above, it has been deemed desirable to develop an apparatus and method directed to maintaining high directionality control of droplet ejections during the printing of image areas with predetermined first optical density requirements, while at the same time relaxing certain constraints which will increase misdirectionality of droplet ejection when printing image areas which have an optical density greater than the first optical density. Such constraints are directed to the time between droplet ejection required for the settling of an ink surface.